Composting has been heretofore practiced for organic waste material such as municipal waste, sewage sludge, livestock farming waste or the like by way of aerobic fermentation. As is well known, basic requirements for ensuring the conventional composting operation consist of uniform heaping of the waste material to a certain height and agitation to be conducted at a specific time interval. However, when composting has to be practiced on a large industrial scale, it has been found that an excessively wide area is required for heaping a large volume of waste material for a long period of time and thereby the conveyance of waste material, the heaping of the conveyed material and the agitating of the heaped layer of waste material are performed only with many difficulties.
When waste material is heaped in a layer to a certain height, the bottom portion becomes densely compressed due to the dead weight of the heaped material, resulting in substantially reduced air permeability of the layer, and aerobic fermentation is positively inhibited in such case.
If the height of the heaped layer is low, the total heat generated by fermentation is small compared to that in a thick layer. However, the radiation of heat or heat loss from the surface of the layer becomes large relative to the total heat because the surface area through which heat is radiated is the same. Therefore, if such relative heat loss becomes large, it may not be able to maintain the temperature of the layer at a value suitable for fermentation.
A higher temperature is preferred since fermentation is accelerated and this is completed in a shorter period of time.
As to composting temperature, it is preferable that fermentation be effected at a temperature in the range of 65.degree. to 80.degree. C., while maturing is effected at a temperature in the range of 40.degree. to 50.degree. C. In order to ensure fermentation and maturing at an increased rate it is necessary to maintain an operating temperature higher than atmospheric temperature at any rate. Thus, it is important that an operating temperature be constantly maintained with a proper and uniform thickness of the layer.
Since effective fermentation and maturing is impeded if the heaped layer of waste material is let alone as it is, agitation of the layer of waste material is usually carried out.
Objects of agitation are generally classified into the following categories:
(1) prevention of excessive compression particularly at the bottom of the heaped layer (preventing generation of aerophobic bacteria) PA1 (2) promotion of fermentation and maturing due to exposure to air particularly when no aeration is provided at the lower part of the heaped layer of waste material.
It should be noted that continuous agitation is not required for all cases and agitation is effected at certain time intervals which are determined by the properties of the materials under composting operation, the degree of fermentation and maturing of the composting materials and the thickness of the heaped layers.
Therefore, the frequency or interval of the agitations applied on the heaped layers of the material may range, just for example, from 2 times per day to one time per 7 days or more. However, the time schedule is not to be limited to such range.
Composting apparatuses and methods of the above-mentioned type are disclosed in U.S. patent application Ser. Nos. 150,652 and 254,278.
The methods and apparatuses disclosed in these applications are found to be useful for treating a large volume of waste material, and the apparatuses are constructed so that material is supplied from one side of a rectangular composting area and moves toward the opposite side while agitation is performed, and then after completion of the intended fermentation and maturing composted material is discharged from the opposite side of the composting area. When it becomes necessary to enlarge the composting capacity of the conventional apparatuses as disclosed in the above applications, the change or modification is to lengthen the material supply side or the span length across the composting area. However, the extent to which the conventional composting apparatus can be lengthened along the material inlet side is limited for economic and practical reasons relating to the structure and mechanical strength of the apparatus. Therefore, it is impracticable to lengthen the composting apparatus in this way. On the other hand, if it is attempted to increase the dimension in the material displacing direction, the distance through which the material is to be displaced becomes quite long.
In case the dimension is increased in the displacing direction, the number of agitation cycles must also be increased in order to move the material from the inlet side to the discharge side, and this results in excess agitation in the maturing zone and promotes undesirable cooling.
For instance, the heaped layer of the material is advanced approximately 1.5-2.5 m in general per one agitation, although each distance depends on the diameter of the paddle used for agitation and the revolutional speed of the paddle.
Also, it is generally admitted in the art that the moving distance of the heaped layer is preferably 1.5-5 m/day for the fermentation stage and 1.5-2.5 m/two days for the maturing stage. It is, therefore, preferable to maintain the moving distance of the layer within the preferable ranges above by taking into consideration effective fermentation and maturing with economical power consumption.
Other typical prior art arrangements are shown in U.S. Pat. No. 3,881,707, U.K. Pat. No. 521,894 and Swiss Pat. No. 496,600, but it is found that none of them is satisfactory to overcome the drawbacks noted above.
That is, U.S. Pat. No. 3,881,707 discloses a self-propelled vehicle for a composting operation, but it is troublesome to supply material to this vehicle for composting and there is a reduction in efficiency of the working area due to the necessity for sparing a certain space between adjacent ridges of the material through which the vehicle moves forward or backward.
Next, U.K. Pat. No. 521,894 discloses a composting apparatus of the type in which waste material to be composted is piled in a group of pyramide shaped piles, but packing down of the material into a mass is promoted at the bottom of the piles and the heat loss from the surfaces of the piles becomes large due to the increase in the total surface area of the piles. Further, it is troublesome to prepare piles or ridges of the material.
Swiss Pat. No. 496,600 discloses a composting apparatus which may be capable of processing a large volume of waste material, but the agitating operation thereof is complex.